A common problem for exterior vehicle lighting is that supposedly sealed lamp housings may actually contain minute leaks (especially large multi-component lamp housing assemblies), such that the interior lamp housing volume does not remain constant. If there are small housing leaks, the air leaks out as its pressure increases due to the lamps heat. When the bulb is subsequently turned off, the internal air cools and the pressure drops. The resulting pressure difference between the inside and the outside draws in fresh air, including water into the lamp. If the lamp is particularly wet at the time, as in a rain storm, the amount of water drawn in can be substantial. This breathing cycle can then lead to the collection of liquid water inside the lamp housing.
The accumulated liquid water in the lamp interior is a primary failure mode of vehicle lamps. Interior water is visible to the customer, either as water sloshing around at the bottom of the lamp housing, or as severe fogging of the lamp lens. The interior water can contact the bulb's glass surface while the bulb is energized. The relatively cool water suddenly touching the hot glass (.about.900.degree. F.) can cause the bulb to catastrophically shatter. Interior water can also degrade (oxidize) the thin metallized reflector surface, thus ruining the lamp's optical performance and cosmetic appearance. Additionally, standing water can corrode other lamp components or lead to the growth of green algae in lamp housing.
It is not clear that a plastic headlamp should be sealed, even if it could be sealed. If reliably sealed, the heating and cooling would still induce pressure changes causing the plastic walls to flex. Flexing the optical surfaces then influences the projected beam. The degree of change in the beam pattern would depend on various factors such as, ambient temperature and pressure, time of operation and so on. Since plastic headlamps are not reliably sealed, and likely should not be anyway, there is a need to properly vent the housing. Venting has its own problems.
While a vent should provide rapid pressure equilibration between the lamp housing's internal and external operating air pressures, it should not allow entry of liquid water or dirt into the lamp housing under normal operating environments.
A related problem with exterior vehicle lamp housings is that the lamp housing interior gets hot upon extended energization of the bulbs (capsules) in the confined interior lamp housing volume. The interior heating profile is typically not uniform, rather the interior lamp housing components directly above the capsules get hot (200-400.degree. F.), whereas the components elsewhere in the lamp housing equilibrate at temperatures of 100-200.degree. F. One of the factors causing the nonuniform interior temperature distribution is the lack of air flow in the lamp housing, and the lack of air interchange between the lamp housing internal and external environments.
There were four common methods used to solve the venting problem on exterior vehicle lamps: 1) the lamp housing was not vented at all (sealed), 2) a drain or labyrinth pathway was created in the lamp housing to allow free flow of air, yet provide a tortuous path to limit liquid water entry, 3) a rubber tube (normally opening downward) was attached over a lamp housing opening (boss) which also allowed air flow, yet provided a tortuous path for water entry, and 4) a hydrophobic membrane with pore sizes on the order of 0.5 to 5.0. was attached over a lamp housing opening, which allowed air to pass yet prevented the entry of liquid water. All of these vent types have been used on various vehicle lamp types including headlamps, fog lamps, signal lamps, combination lamps, and high mounted stop lamps.
With proper lamp housing and vent design, most of these vent types can be successful in preventing water intrusion into the lamp housing. Yet none of these vent types successfully alleviates the high local thermal loading of the interior lamp housing components directly above the energized capsules. The sealed lamp housing allows no air interchange between the internal and external lamp housing environments. The hydrophobic membranes allows only a miniscule amount of air interchange, but not nearly enough to cool interior hot-spots or change the interior air flow currents. While theoretically possible to allow air interchange, the drain and tube vent concepts do not significantly modify the internal convection currents due to the configuration of these vents and their location on the lamp housing. There is then a need for a vehicle lamp housing with an improved air vent.